JOHANNESBURG (Reuters) - South Africa's state prosecutor said on Friday it would go to the top court to challenge a ruling reinstating corruption charges against President Jacob Zuma - drawing accusations from the opposition of a cover-up.

Zuma - whose ruling African National Congress contests local government elections next month - has faced mounting calls to quit from the opposition and even some of his own supporters after a series of scandals and court rulings against him.

Legal pressure mounted in May when the High Court ordered a review of a National Prosecuting Authority (NPA) decision to set aside hundreds of corruption charges against Zuma, calling it "irrational".

The NPA, which has already tried and failed to appeal against the order at the High Court, said on Friday it would fight on as the case raised points of law and other principles, without going into detail.

"The NPA has decided to apply for leave to appeal directly to the Constitutional Court against the judgment," it said in a statement.

The main opposition party said the state prosecutor's decision was an attempt to shield the president and buy him time before the August elections.

"We call on the President and the NPA to dispense with this approach and proceed with the institution of charges," the Democratic Alliance said in a statement.

The charges relate to a major government arms deal in the late 1990s.

The NPA's decision to set them aside in April 2009 cleared the way Zuma to run for president the same month.

At the time, the NPA said it had evidence the charges in were part of a political plot against Zuma.

In April, Zuma survived an impeachment vote after the Constitutional Court said he broke the law by refusing to refund some of the 240 million rand ($16 million) of state money spent on refurbishing his private residence.

In December he was widely criticized for changing his finance minister twice in a week, sending the rand plummeting.

Record unemployment and a looming recession have exacerbated discontent with Zuma's leadership.